1. Field of the Invention
This invention relates to a polymer-dispersed liquid crystal device (hereinafter referred to as a "PDLC device") and, more particularly, to a PDLC device which is based on an ultraviolet polymerizable matrix. Devices according to the invention display a selectively adjustable, variable transmission of specular light as a function of applied voltage. This invention also relates to a method for preparing PDLC devices generally.
2. Description of the Related Art
PDLC devices generally comprise droplets of a biaxially birefringent, nematic liquid crystal material dispersed in a transparent polymeric matrix. PDLC devices are of interest because they can be electrically controlled or switched between relatively translucent (i.e. light scattering) and relatively transparent (i.e. light transmitting) states. This occurs because the liquid crystal droplets exhibit birefringence. As a result, the droplets strongly scatter light when they are randomly oriented in the matrix and the PDLC devices appear translucent. However, upon the application of either an electric field or a magnetic field, the droplets become aligned along the direction of the electric/magnetic field vector and more directly transmit light. Alternatively, the droplets can be thermally stressed to induce alignment.
If the refractive indices of the liquid crystal material and the polymeric matrix are closely matched while in the field-induced, aligned state, the devices appear transparent. Thus, upon the application of an electric field, a magnetic field, or a thermal stress, PDLC devices switch from a state in which they appear translucent to a state in which they appear transparent. Upon removal of the electric field, the magnetic field, or the thermal stress, the devices revert to a translucent state.
PDLC devices are sometimes described as being switchable between opaque and transparent states. Strictly interpreted, the description of PDLC devices in the "field-off" state as "opaque" is not correct. However, the terms "opaque" and "translucent" have apparently been used synonymously and there seems to be no significant misunderstanding regarding the functional appearance of the devices. More accurately, PDLC devices in the field-off, nonaligned state transmit light, but the light is dispersed to the extent that images viewed through the devices appear cloudy or diffuse. That is, the PDLC devices are translucent. Truly "opaque" devices do not transmit light.
PDLC devices have found use as light valves, filters and shutters. The devices have also been used in information display arrangements where it is desirable to have a sharp, rapidly achieved contrast between the translucent and transparent states for addressing purposes such as is required for multiplexing. By "sharp" it is meant that the devices experience a substantial change in the percentage of light incident upon the device which can be specularly transmitted therethrough for a correspondingly small change in the amount of applied voltage. That is, a small change in the voltage (e.g., a change of less than 10 volts) applied to the device causes the device to switch between the transparent and translucent conditions. By "rapid" it is meant that the time required for the device to switch between the transparent and translucent states is very short (on the order of milliseconds).
It is an objective of most presently known PDLC devices to exist in only one of two extreme states (either translucent-off or transparent-on). These devices do not provide a uniform, variable optical transmission or variable grey scale. Furthermore, these PDLC devices cannot be varied and selectively adjusted from, at one extreme, a translucent off-state (corresponding to about 0% relative transmission) to, at another extreme, a transparent on-state (corresponding to about 100% relative transmission) with an infinite number of intermediate, preferably uniform, specular light transmission levels in between. This inability to provide an infinite number of intermediate light transmission levels is believed to be due, in part, to the objective that the devices switch sharply (i.e., over a small change in voltage) between the translucent and transparent conditions. Thus, the devices tend to exist only in these extreme states.
Variable grey scale PDLC devices would be useful in numerous applications. If provided in the form of thin, flexible sheets, the devices could be applied to motor vehicle sunroofs or side windows so that occupants of the motor vehicle could adjust the PDLC device to regulate the amount of specular light passed therethrough. Similarly, the devices could be applied to architectural windows, sloped glazings, skylights, interior glass partitions and the like to provide glare and/or privacy control for occupants of the building.
PDLC devices and methods for preparing them have been described both in the scientific and patent literature. For example, a device which suggests a variable grey scale is disclosed in U.S. Pat. No. 4,749,261 to McLaughlin et al. and assigned to Taliq Corporation. This patent discloses a shatterproof liquid crystal panel which comprises a pair of transparent boundary surfaces formed of glass or plastic with nematic curvilinearly aligned phase ("NCAP") liquid crystal material disposed therebetween. The NCAP liquid crystal material comprises plural volumes of an optically transparent liquid crystal formed in an optically transparent containment medium such as a polyvinyl alcohol or a latex. The volumes of liquid crystal material may be separate from one another, may be interconnected to one or more volumes, or may include both separate and interconnected volumes. The liquid crystal material may be prepared as an emulsion of liquid crystal and containment medium, the emulsion being subsequently dried (i.e., cured). Alternatively, the liquid crystal material may comprise a plurality of individually formed capsules of liquid crystal in a containment medium. The panel further includes a pair of electrodes and a variable element which can adjust the magnitude of an electric field applied to the liquid crystal material. Reportedly, by varying the magnitude of the electric field applied across the liquid crystal material, the extent to which light is transmitted through the panel may be varied.
The NCAP liquid crystal materials of the McLaughlin et al. patent are made by an emulsion or encapsulation technique which is described more fully in U.S. Pat. No. 4,435,047 to Fergason. Emulsion or encapsulation typically involves emulsifying a liquid crystal material with an aqueous phase containing the encapsulating medium, spreading the emulsion onto a substrate, and allowing the aqueous phase to evaporate. Such systems are sensitive to moisture degradation and demand the use of relatively thick, spacer-separated substrates which can be easily coated. For example, FIG. 4 describes a liquid crystal display device which includes a substrate having a thickness of about 10 mils, (including an approximately 200 angstrom thick first electrode), a liquid crystal/encapsulating medium layer approximately 1 mil thick, and an approximately 0.5 mil thick second electrode. Because a water soluble polymer or a polymer emulsified and dispersed in water is employed, the structure presumably has limited water resistance.
The moisture sensitivity of such devices is considered in U.S. Pat. No. 4,992,201 to Pearlman which proposes, as a solution to this problem, that the liquid crystal material be dispersed in a latex medium, the latex medium being obtained by drying a suspension of natural rubbers, synthetic polymers or synthetic copolymers. The liquid crystal/latex blend may be coated onto a substrate and dried.
On the other hand, PDLC devices, such as those disclosed herein involve polymerization-induced phase separation, a technique which offers certain advantages over the emulsion or encapsulation process. Polymerization-induced phase separation is a solvent-free approach which results in the formation of structures which are less moisture-sensitive. Also, polymerization-induced phase separation allows for the production of higher molecular weight matrices which have enhanced structural properties so as to impart certain desired characteristics to the matrix.
In polymerization-induced phase separation, liquid crystal microdroplets spontaneously form in a polymer matrix upon the separation of the liquid crystal and matrix phases. Phase separation is induced by causing the uncured matrix material to polymerize. A polymerization induced-phase separation in which the uncured matrix material polymerizes upon exposure to ultraviolet (UV) radiation is particularly desirable because these systems are easily handled, do not require two-part formulations (as do epoxy-based systems), and because the phase separation kinetics can be readily controlled by adjusting the process parameters.
An early report of polymerization-induced phase separation is found in U.S. Pat. No. 3,935,337 to Taylor. More recently, U.S. Pat. No. 4,728,547 to Vaz et al. disclosed an optically responsive film comprising liquid crystals dispersed in an UV-curable polymer matrix. Liquid crystal/matrix material was applied between a pair of 20 micron (.mu.) silica microsphere-separated glass plates and then exposed to UV radiation. UV-curable polymer matrices include those based on thiol-ene chemistry. FIG. 1 of the Vaz et al. patent suggests that within the polymer matrix, a uniform distribution of equally sized liquid crystal microdroplets is desirable. Reportedly, the liquid crystal microdroplets should be about 0.1 to 10.mu., preferably 0.5 to 1.mu. in diameter. High intensity UV radiation was used to cure the liquid crystal/matrix system (6 seconds of exposure 3 to 4 inches from a 300 Watt/inch mercury discharge lamp). The film may be used for information displays, light shutters and the like, applications for which a variable grey scale would be undesirable.
U.S. Pat. No. 4,834,509 to Gunjima et al. discloses an optical device in which liquid crystal material is uniformly dispersed in a vinyl group-containing matrix that may be polymerized with UV energy. The liquid crystal/matrix blend is disposed between a pair of electrode-bearing substrates. The patent suggests that mechanical spacers (e.g., glass, plastic or ceramic particles) may be desirably employed to carefully control the distance between the substrate electrodes thereby minimizing irregularities in light transmission due to coating thickness. The devices are useful as large area displays, light controllers and light shutters, applications for which a variable grey scale would be undesirable.
U.S. Pat. No. 4,688,900 to Doane et al. and assigned to Kent State University discloses a light modulating material comprising liquid crystal droplets dispersed in an epoxy or a polyurethane matrix. The light modulating material is disposed intermediate a pair of substrates. The matrix is cured in a phase separation process either thermally, upon exposure to UV light energy, or with a chemical promoter. Relatively thick structures in which the boundary layers (substrates) are separated by spacers and in which equally sized spherical liquid crystal droplets are uniformly dispersed in the matrix are provided.
Thermally-cured epoxy-based polymer matrices are also disclosed in U.S. Pat. Nos. 4,673,255 and 4,685,771, each to West et al. and each assigned to Kent State University. None of the aforementioned patents assigned to Kent State University is known to exhibit a uniform, selectively adjustable, variable grey scale but rather are useful in information displays, light shutters, and the like.
U.S. Pat. No. 4,944,576 to Lacker et al. discloses a PDLC device in which microdroplets of a liquid crystal material are dispersed within a photopolymerizable matrix material. The liquid crystal/matrix blend was applied between a pair of spacer-separated, electrode-coated substrates and cured with UV radiation. An electric field, a magnetic field or a mechanical stress is applied during photopolymerization to partially align the liquid crystal microdroplets. As a result of this partial alignment, the PDLC device performs similarly to known devices but with lower threshold and operating voltages. Lower threshold and operating voltages are typically associated with a sharp transition between the translucent and transparent states which is supported by the failure of FIG. 4 (graphical plots of % transmission v. 100 Hz Signal, rms volts (i.e., voltage)) to describe a variable grey scale PLDC device.
U.S. Pat. No. 4,938,568 to Margerum et al. discloses various PDLC devices comprising microdroplets of a liquid crystal material dispersed in a photopolymerizable matrix and applied between a pair of electrode-coated, spacer-separated substrates. By controlling the conditions of photopolymerization, Margerum et al. can create a variation in the size of the liquid crystal microdroplets. Reportedly, several different types of PDLC films may be obtained by spatially varying the conditions of polymerization over the film so that the sizes of the liquid crystal droplets are also spatially varied. In one approach, the exposure intensity is spatially varied by exposing the film through a mask which has a spatial variation in transmissivity. The mask may be at least partially transmissive over its entire area, thereby enabling substantially the entire film to polymerize at about the same time, but at spatially varying polymerization rates corresponding to the spatial variation in mask transmissivity. Alternatively, polymerization may take place in a two-step process by an exposure with the mask in one step, and an exposure without the mask at a different exposure intensity in another step. This technique is based on the observation by Margerum et al. that liquid crystal droplet size may be reduced by increasing the intensity of the UV radiation.
A representative structure is shown in FIG. 3 which illustrates alternating bands of "large" and "small" liquid crystal droplets which repeat from one edge of the PDLC film to the opposite edge. An alternative structure is shown in FIG. 5 which schematically illustrates a variation in liquid crystal droplet size through a PDLC film from one major planar surface to the other. The resulting PDLC devices have reduced operating voltages relative to those previously known. A reduction in operating voltage is typically associated with a sharp transition between the transparent on-state and the translucent off-state. Consequently, this patent does not disclose a PDLC device which exhibits a variable grey scale.
U.S. Pat. No. 4,411,495 to Beni et al discloses a refractive index switchable display cell, the opacity of which may be varied by changing the amplitude of an electric field applied across the device. The cell comprises a preformed, commercial porous filter imbibed with a liquid crystal material. (A similar device is disclosed in "New display based on electrically induced index matching in an inhomogeneous medium") Appl. Phys. Lett. 40(1), Jan. 1, 1982 (pp. 22-24) by H. G. Craighead et al. The preformed filter serves as a spacer and the device is described as "providing a gray scale.")
Interest in PDLC devices has spawned a spate of technical and academic articles. For example, "Response Times and Voltages for PDLC Light Shutters," Liquid Crystals, 1989, Vol. 5, No. 5, pp. 1453-65 by B.-G. Wu et al. notes that the type of polymer matrix can dramatically influence the switching voltage (the applied voltage differential required to transition the PDLC device between the translucent off-state and the transparent on-state). A PDLC device employing a poly(methyl methacrylate) matrix may have a switching voltage of about 200 volts (V), while an identical device reportedly having the same droplet size and shape but using an epoxy matrix may have a switching voltage of 20 V. These observations were based on a system using a liquid crystal material and poly(methyl methacrylate) in a 1:2 ratio by weight. The mixture was applied between a pair of spacer-separated, electrode-coated substrates.
"Droplet Size Control in Polymer Dispersed Liquid Crystal Films," SPIE, Vol. 1080, Liquid Crystal Chemistry, Physics and Applications (1989), pp. 53-61 by A. M. Lackner et al. teaches the formation of PDLC devices comprising liquid crystal droplets dispersed in an UV-curable thiol-ene matrix, the liquid crystal/matrix system being applied between a pair of spacer-separated, electrode-coated substrates. Liquid crystal droplet size was reduced by increasing the intensity of the UV radiation. At an intensity of approximately 13 milliwatts/sq. cm (mW/cm.sup.2), a droplet diameter of about 1.0.mu. was achieved. The PDLC devices are not reported as exhibiting a variable grey scale.
"A Light Control Film Composed of Liquid Crystal Droplets Dispersed in an UV-Curable Polymer," Liquid Crystal, 1987, Vol. 146, pp. 1-15 by N. A. Vaz et al. discloses a PDLC device comprising submicron size liquid crystal droplets uniformly dispersed in an UV-curable matrix. The photomicrograph of FIG. 1 appears to show liquid crystal droplets of substantially equal size. The liquid crystal/uncured matrix material is disposed between a pair of spacer-separated, electrode-coated substrates and cured by exposure to UV radiation of 85 mW/cm.sup.2 intensity (50% uncertainty). PDLC film thickness was typically 27 to 30.mu.. The devices are useful for displays and light shutters but do not otherwise exhibit a variable grey scale. The discussion on pages 6 and 7 of the article suggests that the performance of the device illustrated in FIG. 2 has not been optimized and that it would be desirable to have a sharper transition (i.e., the transition between the translucent off-state and the transparent on-state should occur over a smaller voltage range).
"Morphological control in polymer-dispersed liquid crystal film matrices" by F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications (1989), pp. 24-28 discloses the preparation of a PDLC device comprising liquid crystal droplets dispersed in a polymerizable matrix. The liquid crystal/uncured matrix blend was applied between a pair of electrode-coated, spacer-separated substrates and cured using UV radiation in the range of 60 mW/cm.sup.2. Some of the devices obtained by Yamagishi et al. displayed a "polymer ball" morphology in which domains of a polymeric material are understood to be dispersed in a continuous liquid crystal phase. There is no indication that any of the resulting devices which comprise liquid crystal droplets dispersed in a polymer matrix exhibit a variable grey scale.
It is desirable in certain applications to have PDLC devices which display a variable grey scale. Presently known PDLC devices which suggest the possible objective of a variable grey scale employ emulsion/encapsulation formation techniques; however, these techniques suffer from certain undesirable limitations. The formation of PDLC devices using phase separation and an UV-polymerizable matrix is advantageous. However, presently known PDLC devices which make use of such techniques do not exhibit a variable grey scale. Consequently, there is a need for a PDLC device which exhibits a variable grey scale and which employs an UV-polymerizable matrix.
Moreover, in the presently known methods for producing PDLC devices based on an UV-polymerizable matrix material, the matrix material is typically cured (polymerized) by exposing the uncured matrix material to relatively high intensity UV radiation sources, for example, medium or high pressure mercury or mercury/xenon lamps. Such radiation sources can become quite hot during operation, necessitating the use of elaborate and expensive cooling and temperature control systems. Such radiation sources have also been associated with certain maintenance problems. Accordingly, it would be desirable if PDLC devices could be produced in a method which utilizes relatively low intensity UV radiation to cure the uncured polymer matrix material.